Field of the Invention
Commercial Process Background: Depleted uranium alloys are now extensively used by military forces in nonexplosive penetrators which are fired from guns against battle armor, e.g., by the main battle tanks of the U.S. Army. Such alloys also provide hard, dense material for shielding and weights, e.g., drill collars for oil-well drilling. The large-scale use of these alloys produces material which is now discarded but should be recovered and recycled. This invention relates to methods of recovering uranium alloy from various mixtures from which it is not currently recovered.
The term "uranium alloy" is used throughout this disclosure to describe both deliberately alloyed material and nominally "pure" uranium metal, with its small amounts of alloying impurities. Such uranium alloys are chemically similar, e.g., they all oxidize readily, and, therefore, distinction between pure metal and these alloys becomes chemically unnecessary and confusing in relation to the present invention.
For example, penetrators are machined from rolled rods of depleted uranium alloy; this machining creates scrap turnings which are currently buried as waste in dumps for mildly radioactive material, e.g., at Barnwell, S.C. If such scrap turnings could economically be melted, then rolled into rods, valuable uranium alloy would be saved, and environmental damage from unnecessary radioactive burial would be reduced. However, the machining generates heat, and lubricating water-oil mixtures must be used; the hot machine turnings oxidize heavily both in air and in the water-oil mixtures. The presence of such oxide (a) prevents agglomeration of molten uranium alloy, and (b) ties up uranium alloy in a form which is unrecoverable by simple melting.
Other forms of uranium alloy also become oxidized and economically unrecoverable by current technology. Uranium-alloy penetrators, for example, oxidize when they are fired into the sand of firing pits at test ranges.
Reductions of uranium fluoride by magnesium, e.g., commercial bomb reductions, frequently produce dispersed uranium-alloy product trapped in magnesium-fluoride by-product. Uranium alloy in such dispersions is not economically recoverable by current technology, and such mixtures are discarded and buried.
Because radioactive-waste disposal is expensive and valuable uranium alloy is wasted by current practice, there is need for commercially viable process technology to recover uranium alloy from mixtures with other materials, particularly mixtures comprising uranium oxide, uranium fluoride, or magnesium fluoride. The present invention offers pretreatments of such mixtures to prepare them for uranium-alloy recovery by melting and separation.
Chemical Background: The melting point of uranium, 1133.degree. C., is also substantially that for alloys of interest to the present disclosure. That melting point is far below those of uranium oxides, e.g., UO.sub.2 at 2875.degree.. No molten oxides unreactive with uranium alloy are available to dissolve away these oxides from molten uranium alloy--unless the oxide can be removed, the droplets of molten uranium alloy will not agglomerate to useful billets. Uranium oxide can be reduced by magnesium in molten salt of proper density to sink uranium alloy while floating away the solid magnesium oxide by-product (see Prior Art), thereby achieving separation of uranium alloy with magnesium oxide. However, commercial operations would be simpler if magnesium-containing by-product could be removed as a liquid rather than as solid magnesium oxide. The present invention offers pretreatment to provide such a liquid by-product.
The melting point of magnesium fluoride can be reduced below that of uranium alloy by the formation of molten-salt solution, i.e., by the addition of other salts, several of which are substantially unreactive toward uranium alloy. Stability in the presence of uranium alloy, so long as the temperature is low enough to prevent metal or compound vaporization, is shown by the lower-valence halides of the alkali and alkaline earth metals, and of scandium, yttrium, and the lanthanides. Addition of these materials in some cases reduces the complete-melting (liquidus) temperature of magnesium fluoride solutions several hundred degrees below that of pure magnesium fluoride.
Table 1 lists examples of liquidus temperatures for different mole fractions of magnesium fluoride with the listed compounds. This table suggests the large variations in liquidus temperatures caused by additions of different solutes in different amounts.
For melting at atmospheric pressure, other salts such as those in Table 1 should usually be added to magnesium fluoride: Lowering the liquidus temperature and operating near the melting point of uranium (1133.degree.) reduces the reversal of the magnesium-plus-uranium-fluoride reduction:
Net reduction reaction at the melting point of uranium: EQU UF.sub.4 +2Mg.sub.(liquid) =U+2MgF.sub.2 ( 1)
Net reverse reaction if the temperature is 50.degree. C. above the melting point of pure magnesium fluoride and at atmospheric pressure: EQU 3MgF.sub.2 +2U=2UF.sub.3 +3Mg.sub.(gas) ( 2)
Of course, reaction 1 is the one desired.
Similar reactions apply for the uranium alloys of interest in this invention.
TABLE 1 ______________________________________ Approximate liquidus temperatures in .degree.C. at different mole fractions of magnesium fluoride. Mole fraction-&gt; 1.00 0.90 0.75 0.50 0.25 ______________________________________ Salt added MgCl.sub.2 1261 1200 1100 900 650 CaF.sub.2 1261 1200 1100 950 1175 CaCl.sub.2 1261 1200 1050 950 850 BaF.sub.2 1261 1250 1200 1075 910 LiF 1261 1225 1200 1000 775 NaF 1261 1200 1050 1030 850 NaCl 1261 1200 1100 1050 1050 KF 1261 1250 1100 1070 910 ______________________________________
As the liquidus temperature is reduced further below the uranium melting point, reaction 1 moves more completely to the right, but, of course, the uranium is then solid. One advantage to operating with temperatures below the uranium melting point is that molten magnesium (boiling point 1090.degree.) can be floated at the surface of the molten salt to supply magnesium to produce uranium alloys from uranium compounds which may be present (see Prior Art).
Magnesium oxide, which was noted as being detrimental for the separation of molten uranium alloy and molten magnesium fluoride, can form when magnesium and hot uranium oxide are contacted. Deposits of uranium oxide, e.g., on oxidized uranium alloy, can be converted to fluorides by reaction with gaseous or aqueous hydrogen fluoride at about room temperature, and such replacement of uranium oxide by uranium fluoride can be used as one step of pretreatment for uranium alloy recovery. Other oxides, e.g., oxides of alloying elements, can also be converted to fluorides. The acid attacks substantially only the oxides because the alloy takes on a protective film of insoluble uranium fluoride which prevents further attack. Water and excess hydrogen fluoride then dry away or can be dried away, e.g., by heating in a fume hood. This pretreatment to convert these oxides to fluorides eliminates the problem of thickening of magnesium-fluoride molten solutions which would occur if the oxides were added to the molten solution.
Uranium alloy which is substantially free of oxygen but which comprises fluorides of magnesium or uranium, e.g., reduction products of uranium fluoride-magnesium reaction, can be separated from these metal fluorides in a molten-salt bath of composition such as those indicated in Table 1. Such a molten-salt bath can be supported on a molten-uranium-alloy trap which permits a uranium alloy stream to pour out for recovery and recycle while the salt moves elsewhere to by-product disposal.
Make up of the salt additive will be required to maintain the selected bath composition as magnesium fluoride is added. For example, if the top of the molten-salt bath were maintained at 1050.degree. with a composition of 0.50 mole fraction of calcium chloride, along with floating magnesium present to react with uranium fluoride, one would have to add as many moles of calcium chloride as were added by fluorides of uranium and magnesium.
An often advantageous pretreatment involves reacting the uranium fluoride with magnesium prior to introduction into the melt, e.g., in a preheating furnace. Such reduction recovers uranium in compounds which otherwise would discharge with excess molten salt. Such reduction can be done as the uranium fluoride and any uranium alloy present are heated, and preheating is one way to add necessary heat to the molten-salt bath.
This reduction during pretreatment avoids the need to maintain molten magnesium floating at the surface of the molten-salt bath. However, some magnesium floating on the molten-salt bath may still be useful to prevent back reaction due to loss of magnesium through vaporization. In principle, it is possible to form even molten uranium alloy prior to introducing the mixture of alloy with metal fluoride into the molten-salt bath.